The present invention relates to air guns in marine seismic operations. More particularly, the present invention relates to a compact sleeve air gun that increases the chamber volume without substantially increasing the weight of the air gun.
Marine seismic operations record acoustic waves reflected from the sea floor and underlying geologic formations. The acoustic waves are generated by air guns that release air compressed between 2000 and 6000 psi. The acoustic pulse generated by a single air gun is magnified by additional air guns in a seismic array to produce large acoustic waves. These acoustic waves are reflected from the sea floor and geologic formations and are sensed by recording instruments having transducers for converting the acoustic waves into electrical signals. Thesee signals are recorded and subsequently processed to model the subsurface geologic formations.
Conventional air guns store compressed air in a housing that is released through valved ports in the housing. The released air forms a bubble in the water to create an acoustic pressure pulse. An electrically operated solenoid controls the operation of the valves and release of compressed air. One example of an air gun is shown in U.S. Pat. No. 4,623,033 to Harrison, Jr. (1986), wherein an air gun includes valved ports extending 360 degrees around the air gun housing. The port valve comprises a shuttle that reciprocates axially along the air gun.
U.S. Pat. No. 5,365,493 to Harrison (1994) describes an improved air gun that accelerates the shuttle as the air gun is fired. Another air gun design is shown in U.S. Pat. No. 5,001,679 to Harrison, Jr. (1991), wherein the compressed air is released from the housing by the operation of dual shuttles. The dual shuttle concept increases the efficiency of the air gun and the corresponding acoustic output by expediting the release of compressed air from the housing when the gun is fired.
Conventional air guns incorporate a threaded connection between the gun body and a cap which closes the end of the gun chamber. Because the dimensions of the threaded connection add significant weight, conventional air guns are relatively slender to reduce the size of the weight of the threaded connection, and are relatively long to increase the volume of the firing chamber. Since the relatively longer air guns do not release the compressed air as efficiently as a shorter air gun, conventional designs do not efficiently maximize the ratio of weight to length.
The weight of an air gun is important because flotation devices support the air guns in a seismic array, and paravanes spread the flotation devices and air guns as the tow vessel moves through the water. If the weight of the air guns increases, the flotation devices must similarly increase to support the air gun weight. Because larger flotation devices experience greater drag than smaller flotation devices, larger flotation devices require greater towing energy and further require larger paravanes to maintain the spread of the air gun arrays in the water.
Accordingly, a need exists for an improved sleeve air gun that increases the acoustic pressure generated by an air gun without significantly increasing the weight of the air gun.